Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
3.2.1.1. Water Vapor
Water vapor is present in aircraft exhaust in known amounts because the emission index is specified by the stoichiometry of near-complete fuel combustion (Chapter
7). Water vapor concentrations of a few percent at the engine exhaust nozzle far exceed the concentrations of other aerosol precursor gases. Ambient water vapor
also participates in aerosol processes, with concentrations that vary widely depending on flight altitude and meteorological processes. Because of its abundance and
thermodynamic properties, water vapor participates in nearly all aerosol formation and nucleation processes (e.g., Pruppacher and Klett, 1997).
3.2.1.2. Sulfur Species
Aviation fuels (kerosene) contain sulfur in trace amounts. In the current world market, the sulfur content-hence the EI(S)-of aviation fuels is near 0.4 g S/kg fuel or 400
parts per million by mass (ppm; 1 ppm = 0.0001%), with an upper limit specification of 3 g S/kg (Chapter 7). Of importance for the formation of plume aerosol is the
partitioning of sulfur at the engine exhaust nozzle into sulfur dioxide (SO 2 ) and fully oxidized sulfur S(VI) compounds, sulfur trioxide and sulfuric acid (S(VI) = SO 3 +
H 2 SO 4 ). Most fuel sulfur is expected to be emitted as SO 2 based on combustion kinetics and some observations (Miake-Lye et al., 1993, 1998; Arnold et al., 1994;
Schumann et al., 1998). However, a fraction of the SO 2 can be converted into S(VI) by gas phase chemical reactions with OH, oxygen atoms (O), and H 2 O inside the
engine. The fractional conversion depends on details of combustion conditions, turbine flow properties, blade cooling effects (Chapter 7), and mixing (Chapter 2).
Further oxidation can occur in the plume, where the rate-limiting step is thought to be oxidation of SO 2 by OH to form SO 3 (Stockwell and Calvert, 1983) or liquid-
phase reactions of SO 2 with H 2 O 2 , O 3 , metals (Jacob and Hofmann, 1983), or HNO 3 (Fairbrother et al., 1997). Once SO 3 is formed, the gas-phase reaction with
emitted H 2 O to form H 2 SO 4 is fast (< 0.1 s) under plume conditions (Reiner and Arnold, 1993; Kolb et al., 1994; Lovejoy et al., 1996). Gaseous H 2 SO 4 and HSO4-
(H 2 SO 4 )n (mostly with n = 1,2) ion clusters have been observed in jet exhaust (Frenzel and Arnold, 1994; Arnold et al., 1998a,b).
The chemical lifetime of exhaust OH in the early jet regime is determined by reactions with
emitted NO x and by OH self-reactions, the latter leading to the formation of hydrogen
peroxide (H 2 O 2 ) (Kärcher et al., 1996a; Hanisco et al., 1997). Measurements indicate OH
exit concentrations below 1 ppmv (Tremmel et al., 1998). For an OH concentration of 0.5 to
1 ppmv at the engine's nozzle exit plane and without SO 3 emissions, the OH-induced
pathway alone yields about 0.3 to 1% S-to-H 2 SO 4 conversion in the plume (Miake-Lye et
al., 1993; Danilin et al., 1994; Kärcher et al., 1996a). Model calculations indicate overall S
(VI) conversion fractions in the range of 2 to 10% for various supersonic and subsonic jet
engines (Brown et al., 1996a; Lukachko et al., 1998; Chapter 7), consistent with some
earlier SO 3 measurements behind gas turbines (e.g., CIAP, 1975; Hunter, 1982).
3.2.1.3. Chemi-ions
Figure 3-1: Aerosol and contrail formation processes in an aircraft
plume and wake as a function of plume age and temperature.
A large number of chemi-ions (CIs) are expected to be present in aircraft exhaust because
ion production via high-temperature chemical reactions is known to occur in the combustion of carbon-containing (not necessarily sulfur-containing) fuels (e.g.,
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